1. Field of the Invention
The present invention relates to a rotary connector built in a steering system of a motor vehicle to be used as an electrical connection means for an air bag system or the like, and more particularly to a rotary connector which is of the type that a flexible cable is wound in reverse through an inverting section within a storage or containing section defined between a fixed housing and a movable housing.
2. Description of the Related Art
A rotary connector is made in such a manner that a flexible cable is wound and accommodated in between a fixed housing (a first housing) and a movable housing (a second housing) mounted rotatably with respect to this fixed housing, and is incorporated into a steering system of a motor vehicle to be used as an electrical connection means for an air bag inflater or the like fitted in a steering wheel placed under restrictions on number of revolutions. The aforesaid flexible cable has a belt-like configuration, so-called flat cable, which is constructed such that a base film carries conductors. For the use, the flat cable is wound to make a spiral configuration or wound and subsequently inverted halfway in the direction opposite to the previous winding direction. Of these winding methods, the inversion type can considerably shorten the necessary length of the flat cable. While this inversion type rotary connector commonly uses one flat cable, the increase in the number of conductors accompanying the increase in the number of circuits enlarges the width dimension of the flat cable, with the result that difficulty is experienced to thin the whole rotary connector. For coping with this multi-circuit problem, U.S. Pat. No. 3,763,455 discloses a rotary connector in which conductors are distributed to two flat cables.
FIG. 11 is a plan view schematically showing a structure of a cable reel in the same U.S. Patent. In the illustration, a movable housing 101 with an inner cylindrical wall is mounted rotatably relative to a fixed housing 100 with an outer cylindrical wall, and a first flat cable 103 and a second flat cable 104 are put in a ring-like storage section 102 defined between the fixed housing 100 and the movable housing 101. These flat cables 103, 104 are wound along the outer cylindrical wall of the fixed housing 100 and the inner cylindrical wall of the movable housing 101 in opposite directions within the storage section 102, respectively. In this instance, U-shaped inverting sections 103a, 104a are made at the turning points of the winding direction. In addition, The inner end portions of both the flat cables 103, 104 are respectively connected to cable fetching sections 107, 108 disposed in the vicinity of the inner cylindrical wall of the movable housing 101 to be led through these cable fetching sections 107, 108 to the outside of the movable housing 101. On the other hand, the outer end portions of both the flat cables 103, 104 are respectively connected to cable fetching sections 109, 110 located in the vicinity of the outer cylindrical wall of the fixed housing 100 to be led through these cable fetching sections 109, 110 to the outside of the fixed housing 100. Moreover, in the storage section 102, groups of pluralities of rollers 105, 106 are disposed along its circumferential directions, and the inverting section 103a of the first flat cable 103 is looped around one of one group of rollers 105, while the inverting section 104a of the second flat cable 104 is looped around one of the other group of rollers 106.
In the rotary connector thus constructed, when for example the movable housing 101 is rotated clockwise in FIG. 4 the inverting sections 103a, 104a of the flat cables 103, 104 are shifted clockwise within the storage section 102 by a quantity of rotation smaller than that of movable housing 101, so that the flat cables 103, 104 get into winding and tightening conditions in which the inner cylindrical wall side of the movable housing 101 increases in winding amount. Contrary to this, when the movable housing is rotated counterclockwise, the inverting sections 103a, 104a of the flat cables 103, 104 are shifted in the same directions by a rotation quantity smaller than that of the movable housing 101, so that the flat cables 103, 104 get into rewinding conditions in which the outer cylindrical wall side of the fixed housing 100 increases in winding amount. In these winding and rewinding conditions, the rollers 105, 106 are shifted in the same directions due to the forces from the inverting sections 103a, 104a of the flat cables 103, 104.
In the case of the above-mentioned prior rotary connector, since the cable fetching sections 107, 108 for fetching the first and second flat cables 103, 104 from the movable housing 101 are disposed to be close to each other, the inner end portion of the first flat cable 103 is wound by approximately one turn around the inner cylindrical wall of the movable housing 101 and then connected to one cable fetching section 107, whereas the inner end portion of the second flat cable 104 is not directly wound around the inner cylindrical wall but is wound outside the approximately one turn of the first flat cable 103 wound around the inner cylindrical wall and then connected to the other cable fetching section 108. For this reason, both the flat cables 103, 104 differ in winding condition relative to the inner cylindrical wall of the movable housing 101 from each other, and hence the winding diameter of the first flat cable 103 with respect to the inner cylindrical wall assumes the diameter D of the inner cylindrical wall while the winding diameter of the second flat cable 104 with respect to the inner cylindrical wall comes to a value (D+2t) obtained by adding twice the thickness t of the first flat cable 103 to the diameter D of the inner cylindrical wall, with the result that the winding diameters of both the flat cables 103, 104 differ from each other. Accordingly, the winding quantities or the rewinding quantities of both the flat cables 103, 104 to or from the inner cylindrical wall are different from each other, and hence, when the movable housing 101 moves, the inverting sections 103a, 104a of both the flat cables 103, 104 shift at different speeds within the storage section 102. Thus, for example, when the movable housing 101 is rotated in the winding and tightening direction, there is a possibility that the inverting section 103a of the first flat cable 103 collides against the rear end portion of the roller 106 group to be stopped from movement.